[Collins] Exploding 6146 in KWM-2

[Dr. Gerald N. Johnson]

On Sat, 2006-09-16 at 09:40 -0400, Garey Barrell wrote:
> 
> Dr. Gerald N. Johnson wrote:
> > On Fri, 2006-09-15 at 23:43 -0500, Dave, K2DP wrote:
> >   
> >> Food for thought, anyone have some ideas on why a 6146 would literally explode in my KWM-2 while the rig sits in the receive mode ? That is, other than the fact than the tube date code is "63-30" meaning that it is 43 years old !!!!!! Frankly, at that age, I'm not sure what any tube will do, but I am curious to know if others have experienced a similar type failure. Visual examination of the PA circuit in the KWM-2 and the 516F-2 do not show any visible HV capacitor or high wattage resistor failures. 
> >>
> >> 73 and keep'em glowing !
> >> Dave, K2DP,. St. Louis
> >>     
> >
> > There shouldn't be anything IN a tube to cause it to explode. Its a
> > vacuum tube. My guess is that the tube envelope was cracked or a seal
> > leaked and it got gassy and since it sits with about 900 volts DC on the
> > plate with the cathode grounded, when it got gassy enough it drew enough
> > current to heat the gas which then expanded and made the appearance of
> > an explosion. Did you happen to notice that the getter deposit (that
> > silvery spot on the inside of the glass) wasn't so silvery but had some
> > white portions before it blew up? A definite sign of gas. Also the plate
> > current at idle was probably rising. What's the history of this tube?
> >
> >   
> Bias supply failure???
> 
> 73, Garey - K4OAH
> Atlanta
> 
I don't think so, though I think the bias supply should always be tested
and the selenium diode replaced by a silicon diode.

If the bias had gone away and the radio was in transmit mode, the plate
current would be high, the plates would glow on both tubes and if left
that way long enough would melt the glass and the vacuum would pull the
glass in against the works. Since there's nothing to explode without air
seeping in, I still think the tube was cracked and had enough air to
ionize at 900 volts and so explode.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer